1. Field of the Invention
The present invention is directed to an optical system for illuminating and imaging a reflective light valve, and more particularly, to systems having compact, lightweight, and efficient frontlighting for polarization-based reflective light valves and backlighting for transmissive light valves used, for example, in miniature displays.
2. Discussion of the Prior Art
Typically, conventional miniature displays, such as head mounted displays (HMDs), are based on miniature cathode ray tube (CRT) or transmission-based liquid crystal light valve technology. The CRT-based systems are bulky, expensive, and heavy, and primarily used for military helmet-mounted applications. This technology is not suitable for lightweight, compact personal displays.
Transmission-based liquid crystal (LC) technology is the preferred technology for these portable miniature displays today. Although appropriate for the low resolution displays currently available, such as 640.times.480 pixels, this transmission-based LC technology is not adequate for high resolution miniature portable displays.
A transmission technology based display requires a clear aperture for transmission of light through the display. A transparent substrate is also required which incorporates all the display driving circuitry (such as active matrix circuitry). Typically, the driving circuitry uses amorphous silicon on glass technology or poly-silicon on quartz technology. The requirements of transparent substrate, clear aperture, and display control circuitry limit the minimum size of the display panel, thus preventing further display size reductions. To achieve smaller size display panels, reflective liquid crystal (LC) light valves are used.
Reflective liquid crystal light valves do not have the size limitation of transmission-based LC light valves. For reflective LC light valves, using crystalline silicon CMOS technology, the active matrix driving circuitry can be fabricated on 10 micron pixel dimensions or smaller. Furthermore, by using reflection liquid crystal devices, the requirement for a clear aperture in the display panel, needed for transmissive LC devices, is dispensed with. Instead, the reflective device incorporates a mirror array that is fabricated over the underlying CMOS circuitry. In this case, the entire surface of the device is available for display aperture. Thus, the pixel size is only limited by the CMOS technology required to fabricate the drive circuitry, which today is less than 10 microns per pixel. The functioning reflective display panel is completed when the liquid crystal and top glass are assembled over the mirror array. Thus, miniature high resolution displays can be fabricated using silicon-based reflection liquid crystal devices. However, reflection-based light valves, such as liquid crystal (LC) spatial light modulators (SLMs) have complex illumination requirements. In the reflection mode, the SLM must be illuminated and imaged from the same side. A simple backlight structure typically used in transmission-based displays is not directly applicable for reflective SLMs.
The illumination system provides normal incident illumination onto the SLM. This requirement causes the illumination system for both reflective and transmissive SLMs to increase the size and thickness of the display. Accordingly, there is a need for an SLM illumination system that is compact, yet highly efficient.